Semiconductor light sources, including: light-emitting diodes (LED), infrared-emitting diodes (IRED), edge-emitting laser diodes (LD) and vertical-cavity, surface-emitting lasers (VCSEL), are used in most data-transmission and storage systems and in many sensors for measuring gas and fluid properties and detecting proximity and distance. Light emission is stimulated by passing an electrical current through the semiconductor. The efficiencies of these sources, expressed in Watts (W) of light power per Ampere (A) of excitation current (for one W of output at 25° C.), range from about 0.03 for LDs to 0.2 for VCSELs. Light-emitting diodes and IREDs emit incoherent light from all sides of the semiconductor chip and therefore required reflective packaging and collimating optics to direct the emitted light toward transmission devices or fluid samples. A typical laser diode has a semiconductor chip with reflecting front and back facets. Lasing causes emission of coherent, polarized light from both facets in elliptical beams with typical divergences of 120° (full width at half maximum intensity, FWHM). Approximately 10 to 50% of the generated light passes through the back facet to a monitoring photodiode (MD) unavailable for data transmission or sensing (e.g., purposes other than light power control). The light beam from an edge-emitting laser diode is an elliptical cone and a special optic is required to circularize and collimate it. A VCSEL emits a roughly circular, coherent, polarized light beam, with divergence of about 6 to 19°, depending upon the manufacturer, from an array of Bragg reflectors contained in a 5 μm to 20 ∞m on the semiconductor chip. A VCSEL requires only an inexpensive beam-forming optic (BFO) to produce a round collimated beam.
The amount of light a semiconductor source emits depends on the current passing through the device as well as its temperature and its age. Typically the efficiency (W×A−1) will decline with temperature and age. The temperature effect is expressed by the temperature coefficient (t.c.) which is the percent change in light output per unit change of device temperature. The temperature coefficients of semiconductor light sources range from about −1 to −0.5% per degree C. The temperature coefficient can be measured and compensated whereas aging effects are random and unpredictable. Aging typically reduces LED brightness by about 5% per decade of time. Various methods are employed to control the radiated power from semiconductor light sources. One common method is automatic power control (APC) wherein the energizing current to the semiconductor light source is controlled in accordance with a light intensity generated by the semiconductor. In another method, the energizing current can be regulated in accordance with a temperature sensed in the vicinity of the semiconductor.
In sensors such as fluid property sensors, accurate control of optical power incident on a fluid sample is critical. For example turbidity sensors such as that disclosed in U.S. Pat. No. 4,841,157 have light-emitting diodes (LED) or infrared-emitting diodes (IRED). The light-generating current is controlled by a temperature sensor located close to the source. This scheme can reduce the temperature coefficient effect of an LED or an IRED to a few hundredths of one percent per degree C. but it cannot compensate for diminished brightness with device age.
Laser diode light sources often employ monitoring photodiodes (MD), which sense a portion of the light emitted from the LD to control the current to the light-emitting semiconductor. The MD is positioned to measure light intensity from the back facet to control the drive current and thus light emitted from the front and back facets of the chip. This APC system works exceptionally well as long as the ratio of the reflectivities of the front and back facets remains the same. A shift in this ratio as a result of thermal damage, however, will produce power-control errors. Moreover, internal and external reflections, and ambient light create spurious MD photocurrent noise in the APC circuit.
Light-emitting diodes with automatic power control by sensed light intensity are used in telecom systems and sensor applications. A portion of the light can be monitored by an MD mounted adjacent to the chip. While most of the light impinging on the MD comes directly from the LED chip, the MD receives some light reflected from the package and some ambient light passing into the LED package through the window glass. The light received by the MD is converted to photocurrent and used by the APC circuit to control the quantity of emitted light. The reflected and ambient light is an error in the APC system and in daylight, the error can be substantial.
Typical APC VCSELs have an MD mounted adjacent to the laser chip to measure reflection from the window glass or lens. This efficient scheme uses less than 10% of the total radiated power for automatic power control and delivers the rest to a light-transmission device. The automatic power control of a VCSEL is subject to error from ambient light illuminating the MD and from changes in power level and device temperature. This error can be large in sensor applications where the MD is exposed to ambient light. The spatial pattern of radiant intensity in a VCSEL beam varies from one device to another and changes with changes in power level and device temperature.
There exists a need for an APC light source which has improved power-control accuracy, which is efficient in use of electrical power to produce light, which is insensitive to ambient light, device age and changes in device temperature, and which uses simple collimating optics.